GH4169 superalloys are widely used in aerospace engines and other high-temperature components. Powder recycling of the GH4169 alloy during selective laser melting (SLM) can significantly reduce the preparation cost and shorten production cycles. However, the components formed by the SLM using recycled alloy powders exhibit differences in microstructural defects and performance behavior because of changes in the size distribution, shape, uniformity, and composition of the powders. This study investigates the effects of the microstructure, defects, and particle size distribution of GH4169 alloy powders after different recycling times on the microstructure, phase distribution, tensile behavior, and deformation mechanism of formed parts in a heat-treated state.

The GH4169 alloy powder prepared via argon atomization is used in the SLM forming process. The powder is printed and reused for 0‒13 times without adding the newly prepared powder. The large-sized inclusions and support residues are removed by using a 100 μm powder sieve. The specimens are defined as 0th, 6th, 10th, and 13th specimens, according to the number of times the powder is recycled, as shown in Table 1. The 0th, 6th, 10th, and 13th specimens are heat-treated after SLM formation, using the heat-treatment schedule shown in Table 2. Finally, after sample preparation and polishing, scanning electron microscope (SEM) and transmission electron microscope (TEM) photographic analyses are performed.

After multiple powder recyclings, the powder still exhibits good overall degree of sphericity, but the powder morphology changes with an increase in the usage time. The number of defective powders, such as satellite powder and irregular particles, is relatively small among powders with fewer recycling times (0 and 6), as indicated by the powder particles marked by the dashed circle in Fig.1. However, as the recycling time gradually increases, the number of satellite balls in the powder significantly increases in the 10th and 13th samples. Some particles even have 2 or 3 layers of irregular powder coated on their surfaces, which results in an increase in the powder surface roughness and a decrease in flowability, thereby leading to the formation of unmelted pores and micropores in the heat-treated samples (Fig.4). After treatment, the nanosized γ″ and γ′ strengthening phases as well as residual Laves phases exist in the matrix. Moreover, nanosized δ phases and carbides exist at the grain boundaries. As the powder recycling time increases, there is a slight decrease in both the strength and plasticity of the alloy. Each reaches its lowest value in the 13th sample (Table 4) at room temperature (RT) and 650 ℃, which is mainly attributable to the increase in the content of pore defects in the alloy. However, in the 6th sample, the performance reaches its peak, with an ultimate tensile strength (UTS) of 1430.00 MPa, yield strength (YS) of 1318.70 MPa, and elongation of 22.00% at RT. At 650 ℃, the performance has a UTS of 1205.00 MPa, YS of 1130.00 MPa, and elongation of 24.00%. The tensile fracture mode of all specimens at RT is a mixture of cleavage fracture and microporous aggregation fracture, and microporous aggregation fracture is observed at 650 ℃. After powder recycling, the content of porosity and crack defects significantly increases, especially at 650 ℃, where micropores can directly merge to form cracks and thereby damage the properties of the alloy.

In this study, the average particle size of the powder increases, the surface roughness increases, and the fluidity decreases after powder recycling, resulting in pore defects in the heat-treated specimens and leading to the impairment of mechanical properties of the alloys. However, the fracture mode and deformation mechanism are unaffected. The tensile deformation mechanisms of the alloy at the two selected temperatures are the nanoscale δ phase, carbides, Laves phase, and γ″/γ′ hinder dislocation movement. At 650 ℃, micro-twinning appears, synergistically strengthening the strength and plasticity. The main sources of strengthening and toughening are the precipitation strengthening, dislocation strengthening, and fine grain strengthening.